The need for controlling the value of the magnetic field within a specified volume exposed to a significant ambient magnetic field, which may also be varying, arises in a number of situations. One such situation, for example, is a cathode ray tube (CRT) monitor in which the electron beam of the cathode ray tube gun is deflected magnetically to scan with precision the phosphor face of the monitor. The precision requirement is even more demanding with a color screen which requires electrons to strike the phosphor to produce specific colors by passing through holes in a shadow mask at precise angles to strike only the intended color phosphor. There have been a number of approaches employed to compensate for the effects of the extended ambient magnetic field in the operation of such monitors. Of course, one straightforward approach is the use of passive magnetic shielding. However, this is limited both as to the magnitude of field it is practical to shield, and also by the inability to surround the entire volume with shielding.
Another approach has involved the use of electrical coils positioned in locations around the monitor, which coils are energized to produce appropriate magnetic fields. In some instances, such as that described in U.S. Pat. No. 2,925,524 the fields so generated are preadjusted before the monitor is placed in operation. Another approach is described in U.S. Pat. No. 4,380,716 in which patterns at the corners of the phosphor screen are generated by a specific portion of the electron beam output path. Changes in these patterns due to changes in the ambient magnetic field are sensed by optical sensors, which in turn control the current flow through the coils, providing only axial correction. Still another approach (described in U.S. Pat. No. 3,757,154) utilizes magnetic sensors placed in a bridge to control the current flow through correcting coils. This arrangement is, however, open loop and, accordingly cannot produce magnetic fields under positive control to accomplish the compensation function.
It is therefore an object of the present invention to provide active magnetic field generators, for nulling the magnetic field within a specific volume within an ambient external field, with the energizing current for the generators being controlled by the value of magnetic field measured by a number of sensors, all included in a feedback control loop.